The present invention is generally concerned with electronics and more specifically concerned with multiline switch systems for use with switching at least two different frequencies wherein "fail operational" capability is desired for both frequencies. Even more specifically, the present invention is directed to multiline switching repeater stations situated in microwave networks.
Conventional multiline switches provide protection for either IF or baseband signal paths in a microwave radio system. Multiline protection switches are used to protect against interruptions or degradations to microwave working channels which may be caused by radio equipment failures or severe propagation fading. When one of the n working channels fails, the working channel traffic is automatically transferred to the spare channel. After the working channel returns to normal conditions, the traffic which was previously switched to spare is restored to its normal working path. The spare channel is now available for protection against other failures which may occur on any of the working channels. Baseband protection switches are used at the terminal ends of the microwave system since baseband frequencies are required as a normal form for message or video traffic interfacing with other equipment. IF (intermediate frequency) protection switches are used at intermediate repeaters in order to subdivide the system into switching sections. Having multiple switching sections increases the reliability of the system since independent switching can occur in each section to protect against multiple failures in the total microwave system.
IF switches are used at switching repeaters in lieu of baseband switches because the normal signal, connected through the station, is IF. The use of IF switches avoids the use of remodulating equipment in the radio equipment. It is important to avoid remodulating where not necessary since remodulating equipment introduces noise into a microwave channel and degrades the transmission characteristics of that channel. It is thus desirable to minimize the number of times signals are passed through remodulating equipment in a long haul microwave system.
Multiline baseband switches and multiline IF switches are normally formed by two different sets of equipment. In other words, one multiline switch contains only baseband switches and a further one or more multiline switches contain only IF switches. The switching elements are different because IF and baseband signals are formed at substantially different signal levels and frequencies and the switch for switching baseband signals will not provide satisfactory signal passing characteristics to IF signals and vice versa.
In prior art U.S. multiline baseband switches, it is conventional to supply only one spare radio transmission path and accordingly one spare switch. As outlined, above, however, this switch is of the same type as all the rest of the switches in the switch equipment. This is true whether the switch is a baseband switch or an IF switch. In Europe and some other foreign countries, it is conventional to have two spare radio channels and thus there are two spare IF or two spare baseband switches. Again, however, all known prior art uses the same type switch throughout a given piece of equipment. Where there are two spare systems, (in Europe) two working channels may be switched to the two spares simultaneously. It will be realized, that in the Unites States only one spare radio path is allowed in a microwave system by FCC regulations.
It should be further realized that at IF switching repeaters, it is often required that access to one or more of the IF working channels be provided so that baseband traffic can be dropped and inserted into the microwave system. Baseband drop and insert has been accomplished in the prior art by using redundant frequency modulated receivers (FMR's) and frequency modulated transmitters (FMT's) which are connected to the channel (or channels) having the drop and insert requirements. Redundant FM equipment, with sensors, switches and switching logic is required in order to protect against circuit outage due to equipment failure in the drop or insert signal path. Those channels having no drop and insert requirements are patched between the IF receive switch and the IF transmit switch at the repeater station. The advantage to this approach in the prior art is that channels which are not involved in the drop and insert are maintained at IF frequencies thereby minimizing the distortion and degradation of the transmission characteristics of the channel.
Another approach used in the prior art in implementing a drop and insert repeater station involves equipping the repeater with back to back baseband, multiline switches instead of IF switches. This approach eliminates the need for separate FMT and FMR equipment on those channels requiring drop and insert capabilities. The disadvantage to this approach is that all channels must be equipped with FMR and FMT equipment in order to convert the signals on all channels to baseband. This approach adds FMR and FMT equipment to those channels not requiring drop and insert capabilities with the resulting disadvantage that performance degradation is introduced into those channels not requiring drop and insert capabilities.
The present invention overcomes the difficulties of the prior art by using two separate spare switches with a single radio transmission channel wherein one of the spare switches is a baseband switch and the other is an IF switch. Additionally, the multiline switch equipment is configured such that the IF and baseband switches have interchangeable connection capabilities. Thus, any given multiline switch can be used in any combination of baseband and IF switches. If there is degradation of performance in any given channel, it can be switched to the appropriate baseband or IF spare switch and the spare IF channel may be utilized to provide the signals. In other words, baseband drop and insert is accomplished by using a multiline combination switch which provides the capability of providing any working channel slot with switches for either IF or baseband signals. This approach also uses one set of FMR and FMT equipment on each working channel having drop and insert and one set of FMR and FMT equipment on the spare channel which provides protection by the multiline switch. (i.e. external protection is not needed.) Thus, the total amount of equipment required to accomplish a given combination of repeater and drop and insert functions is substantially less than in the prior art.
It is therefore an object of the present invention to provide an improved multiline dual frequency switch.